The present invention relates generally to the reclosable plastic bag art and in particular to string zipper used in a method of manufacture of such bags.
The common reclosable plastic bag is provided on one bag sidewall with an extruded profile that interlocks with a complementary mating profile on the opposite sidewall. The profile configurations or method of attachment is such that a greater force is required to open the bag from within than from without. As a result, the bag is rendered relatively easy for the user to open while being capable of resisting internal forces when filled that otherwise would cause the bag to "pop" open.
The force differential required to provide the aforementioned easy-from-outside, hard-from-inside opening operation of the reclosable bag may be obtained by properly configuring the profiles, such as by providing asymmetric male and/or female profiles (for example as disclosed in U.S. Pat. No. 3,198,228) or by providing a post or other means to make opening the bag easier from the outside than the inside (for example as disclosed in U.S. Pat. No. 4,736,451). In either case, the orientation of the zipper on the bag walls is critical to insure that the easy open side is directed toward the bag opening and away from the bag interior.
Reclosable plastic bags are commonly formed of a sheet of plastic material on which the profiles are integrally extruded or to which separately extruded profiles are bonded. The sheet material is formed into a tube, folded flat with the profiles joined and transverse seals are formed to provided the sides for adjacent bags.
Where the profiles are formed integrally with the sheet material their orientation is properly predetermined by the position of the profile dies. However, where the profiles are separately formed and then attached to a sheet, extreme care must be exercised to insure the proper orientation of the profiles. The zipper to be applied to the sheet may be provided with base flanges adjacent to one or both of the profiles or the profiles may be flangeless, providing a so-called "string zipper". A typical string zipper construction is depicted, for example in U.S. Pat No. 5,276,950. In either case, the zipper is usually shipped to a bag converter wound on a spool which is unwound at the zipper attachment equipment station. The zipper attachment equipment may be a stand-alone device or part of a bag fabricating or form, fill and seal machine. In either case, the zipper is usually provided with the profiles already engaged and the profiles remain engaged throughout the bag forming operation until the bag is first opened by an eventual user.
For a conventional reclosable storage bag or formed, filled and sealed food package, the base of the profiles and hence the width of the zipper is on the order of 0.250". Thus, the differences between the easy and hard open sides is quite minute and difficult to detect. The zipper is commonly wound on a spool of sufficient length to provide for between 2 and 3 hours of continuous bag production, after which the spent spool must be replaced with a new spool. Thus the spools are commonly changed several times during a normal production shift. Since the zipper is commonly formed of polyethylene or similar plastic material, it is easily twisted. Thus, during each change of the zipper spool there is the danger of the zipper spool being misoriented and, even if the spool is properly oriented there is the further danger of the zipper coming off the spool twisting prior to being attached. In either case the zipper could be applied with the easy-open side facing the bag interior and the reversal might not be detected until after the bag is filled and first opened by a consumer. This problem is exacerbated by the fact that in most form, fill and seal operations a tamper proof web spans across the lips of the bag. When in place, the web tends to hold the profiles closed even if they are reversed and subjected to forces that otherwise would pop the profiles open. However, once the consumer cuts the web, if the profiles are reversed the defect in the bag becomes evident by rendering the bag hard to open from the outside and having a tendency to pop open from the inside.